Finding Extrasolar Planets

Astronomers have believed for decades that planets are plentiful, simply because the birth of a star is inevitably accompanied by leftover material — a messy residue of gas and dust that can turn into small, orbiting worlds.

But to actually find planets around stars is tough. You can't simply point a telescope in the direction of a nearby star and hope to see its planets. The orbiting bodies are too dim and too close to a blinding light source (their sun). To grasp the full existential challenge of the problem, imagine trying to see a marble located 30 yards from a light bulb at a distance of 10,000 miles.

Despite these daunting difficulties, astronomers have found extrasolar planets (planets outside of our solar system that orbit stars other than our sun) — not by picking them out in photos, but by

Measuring the slight dimming caused when they pass in front of their sun

Carefully monitoring the slight dance their sun makes because of their presence

The first technique (dimming) takes advantage of the fact that if another solar system is by chance oriented the right way, the planets will — once per orbit — cross in front of their home star as seen from Earth. The mini-eclipses obviously reduce the brightness of the star, if only for a few hours. The dimming is slight: only about 1 percent, even for a hefty Jupiter-sized world. An attentive astronomer with good equipment, however, can notice the difference.

Astronomers have found a few planets with this so-called transit technique, but most of the alien worlds discovered since 1995 have been uncovered by the second technique: measuring the small motions of the host stars.

Planets and stars orbit their common center of mass, and this arrangement means that both objects move. As they orbit around under the influence of their mutual gravitational attraction, the star pulls on the planet, making the planet move, and the planet pulls on the star, making the star move. The planet has much less mass than the star, so the so-called reflex motion of the star usually isn't much — perhaps only 50 miles an hour (compared with the planetary motion, which may be 10,000 miles an hour or more). But by using sensitive spectroscopes — devices that break up the incoming light into its various colors, like a prism — mounted on large telescopes, astronomers can hunt for the small Doppler Effect that the slow stellar wobble produces in the star's light. And they've already managed to find more than 100 stars whose lazy dancing betrays orbiting planets.

The following sections cover several interesting extrasolar planets and plans for continuing the search.

51 Pegasi's hot partner

The credit for discovering the first extrasolar planet belongs to two Swiss astronomers, Michel Mayor and Didier Queloz, who announced their find in the fall of 1995. The discovery caused a great deal of consternation in the research community, mainly because the new planet whips about its star (51 Pegasi) at a breakneck pace. A complete orbit takes only about four days. Consequently, astronomers deduced that the planet orbits a trifling 5 million miles from its host star, or about eight times closer than Mercury is to the sun. The findings also imply that the temperature of the world is roughly 1,000 degrees Celsuis. The size of 51 Pegasi's stellar wobble indicates that the planet's mass is at least half that of Jupiter. For obvious reasons, astronomers soon dubbed the new planet a hot Jupiter.

In the decade following the discovery of 51 Pegasi's warm little world, astronomers have found new planets at the rate of more than one a month, nearly all with spectroscopic measurement of Doppler shifts. Quite a few of the new worlds are also hot Jupiters — massive planets that hug their home stars tighter than a loving mom.

But astronomers don't believe that any of these hot and heavy worlds were born in their present, toasty orbits. Large planets have a far easier time forming in the dim suburbs of a solar system. The colder temperatures and endless expanse of material in these nether regions encourage the rapid conglomeration of icy debris into large worlds. But upon birth, the planets' interactions with the leftover debris may cause them to wander from home and gravitate inward to the hellish domains of their scorching home stars.

Still, most of the newly discovered planets don't orbit their stars as tightly as the planet of 51 Pegasi. This is good news for anyone who likes to think that there may be other solar systems in the universe like our own. Many of the first extrasolar planets scientists found are hot Jupiters, which caused some people to worry that our solar system (where the big planets like Jupiter are far from the sun) is freakish and rare. But as the search continues, astronomers are finding more and more extrasolar planets that have orbits similar to the planets of our solar system.

No one is quite sure what prevents hot Jupiters from going "all the way" and careening into their host stars. One possibility is that these planetary heavyweights raise waves of hot gas on the star's outer surface, and the gravitational effects of these tides halt the planet's inward spiral. But that's still a theory, and astronomers candidly admit that both the birth and ultimate fate of hot Jupiters are phenomena that we don't yet understand.

The Upsilon Andromedae system

In 1999, Geoff Marcy, Paul Butler, and other collaborators (who have discovered many of the new planets detected since 1995) added to the planet-finding excitement by claiming that not one, but three large planets are in orbit around the star Upsilon Andromedae. The group made this discovery through careful analysis of the star's subtle wobbling motions.

Upsilon Andromedae, an F-type star 44 light-years from Earth, thus became the first normalstar (a shining, nuclear furnace type of star) other than the sun known to have a genuine solar system. The planets themselves are hefty, weighing in at greater than 0.7, 2.1, and 4.6 times the mass of Jupiter. They don't all hug the star, however: The outer two planets have orbits comparable in radius to those of Venus and Mars.

Continuing the search for planets suitable for life

Although it reassures people who search for extraterrestrials to know that E.T. has plenty of homes to phone, the new planet discoveries are also a bit disconcerting. After all, hot Jupiters (or, for that matter, cold Jupiters) aren't likely places for biology to cook up, because water on such worlds would either boil or freeze, and liquid water is what we think that all life — including alien life — requires. If these oversized planets are typical of the galaxy's complement of worlds, we shouldn't expect much cosmic company.

But such a scenario is unlikely. The technique that scientists use to find most of the new planets — looking for wobbles by taking advantage of the Doppler Effect in the light from stars — is best for uncovering giant worlds that orbit close to their home stars. You can compare the search for planets so far to a reconnaissance of the African savannas from a helicopter. You can see the elephants and rhinos, but you miss the mice and mosquitoes. Scientists find big planets because we can find big planets. Small worlds are probably plentiful, but until researchers build some new types of telescopes, discovering the small guys is difficult.

In 2007, NASA plans to launch the Kepler mission: a space-based telescope whose task is to find out if small planets really are as common as phone poles. It will stare for four years at a patch of sky containing 100,000 relatively nearby stars, hoping to catch sight of the periodic dimming caused by encircling worlds. The expectation is that Kepler will find many dozen Earth-size worlds with the transit technique, but of course we have to wait to see what really happens.

If planets the size of Earth are abundant, the next step is to find out if any of them support life. The answer may be within reach if proposed new space telescopes are built — instruments such as the Terrestrial Planet Finder (NASA) or Darwin (European Space Agency). These high-flying, high-tech scopes, which researchers hope to launch by 2010 or 2012, would be able to actually catch some of the light from extrasolar planets and, with a bit of simple spectral analysis, determine what compounds make up their atmospheres. If scientists find a lot of oxygen or methane in the air of some far-off world, we may have good reason to suspect the presence of life. Needless to say, building space-based telescopes is much easier than sending a Federation Starship out on a reconnaissance mission.

If you want to know the latest and greatest in the search for extrasolar planets, you can find the facts at the Extrasolar Planets Encyclopaedia, which also has links to many other related sites.